It is allowed for financially able children to perform `Umrah on behalf of their parents or any other Muslim. However, if they want to perform `Umrah on behalf of parents and themselves in a single visit, then they have to perform `Umrah for themselves first, and then make Tahleel (removing the ihram attire) after every `Umrah completed, be it for themselves or on behalf of others by shaving or clipping the hair and then going to Wadi At-Tan`eem to perform ihram from the beginning.”
Allah Almighty knows best.